1. Field of the Invention
The present invention relates to an electromagnetic induction heating apparatus, and more particularly, to an electromagnetic induction heating apparatus which is suitable for the electromagnetic induction heating of a metal strip.
2. Description of the Prior Art
Of all known methods of continuous heating a flat metal strip on line, electromagnetic induction heating has become popular recently owing to its high heating efficiency and excellent controllability of the heating atmosphere.
FIGS. 1 to 4 show one example of such electromagnetic induction heating apparatus (as disclosed in, for example, the specification of Japanese Patent Publication No. 40840/1983, and Light Metal Age 1982-Vol. 40, Nos. 11, 12, Pages 6-11).
In the apparatus of the type shown in FIGS. 1 to 4, a metal strip 2 to be treated is formed into a strip of a predetermined width, and is fed in the direction shown by arrow A by roller mechanisms 4 and a roller-rotating portion (not shown) which actuates the roller mechanisms 4. Electromagnetic portions 6 and 8 are held opposite to each other by support mechanism (not shown) above and below the metal strip 2 as viewed in FIG. 1 at a predetermined spacing, thereby forming a passage through which the strip to be treated is conveyed. The electromagnetic portions 6 and 8 are composed of four electromagnets 6A and 8A, respectively, forming four pairs of electromagnets. The longitudinal direction of each electromagnet 6A or 8A is aligned in the direction of movement of the metal strip 2, and the electromagnets 6A and 8A respectively have a core 10 and 12. The cores 10 and 12 each have a substantially comblike configuration as seen from the side, are rectangular in plan view, and are wound with coils 14 and 16, respectively, as shown in FIG. 1. When an alternating current is passed through the coils 14 and 16, transverse magnetic flux .phi. of an instantaneous value is therefore generated perpendicular to the metal strip 2 as shown by the dotted lines in FIGS. 1 and 2, thereby inducing eddy currents in the metal strip 2 as shown, for example, in FIG. 3, and the metal strip 2 is heated very rapidly by resultant of Joule heat.
Heat shielding materials 18 for shielding the heat radiated by the heated metal strip 2 are provided. Each of the electromagnets 6A and 8A contains a cooling pipe (not shown) which prevents the electromagnet from overheating.
In the apparatus arranged in this manner, an aluminum coil material which has a thickness of between, for example, 0.25 and 2.0 mm is moved at a speed of between 5 to 200 m/min, so that an alternating magnetic field of 60 to 400 Hz is applied to it to enable continuous heating.
In such an apparatus, it is known that the distribution of density of eddy currents J generated in the metal strip 2 and of the temperature of the heated metal strip 2 depend in a complicated manner on the frequency of the alternating magnetic field, the relative difference between the width of the metal strip 2 and that of the electromagnet, the distance between the opposing electromagnets, the desired temperature of heating, the thickness of the metal strip 2 to be heated, and other factors. Of all these factors, relative difference between the dimension of the metal strip 2 in the direction perpendicular to that of the movement of the metal strip (hereafter referred to as the "strip width") and that of the electromagnet as viewed from the side of the strip may be attributable to the local generation of regions of highly dense eddy currents J (look at the shaded portions B in FIG. 3: these portions have a higher eddy currents density than those of the centrally-located shaded portions B') at the two edges of the metal strip 2 which run parallel to the direction of movement of the metal strip 2 (hereafter referred to as "strip edge portions"). These local hot spots mean that temperature is distributed unevenly in the metal strip, as shown in, for example, FIG. 4 (A) (in which, since the temperature distribution is substantially symmetrical with respect to the center of the strip, only one side of the strip is shown. All temperature distributions referred to hereafter are shown in the same manner). Such extreme temperature differences at the strip edge portions can cause faults during the heating treatment, including a reduction in the yield of metal strip due to the non-uniform disposition of grains of the metal strip as viewed from the side, and countermeasures must be taken to cope with such faults.
To obviate this problem, the electromagnets 6A and 8A in the known electromagnetic induction heating apparatus are each provided with a member for adjusting the magnetic flux density (generally called a flux modifier, and hereafter referred to as an "FM member") 20. The FM members 20 are mounted in an FM member moving frame body (not shown), in which they are separated from each other at predetermined intervals which correspond to those of the electromagnets 6A and 8A, and are disposed in such a manner that they can be slid against both edges of each of the electromagnets 6A and 8A which are closer to the metal strip 2 by a driving device (not shown) mounted in the FM member moving frame body. A distance l (see FIG. 2) between the edges of the FM members 20 and those of the electromagnets can be finely adjusted in accordance with the strip widths. Thus, the distribution of magnetic flux at the strip edge portions is adjusted in accordance with the strip widths, thereby minimizing the temperature differences generated by the uneven distribution of magnetic flux.
Other attempts to ameliorate the abovedescribed uneven distribution of temperature at the strip edge portions have been proposed in the specifications of, for example, Japanese Patent Laid-Open Nos. 1339/1978, 1614/1978 and 92428/1985.
Of all of the above-described prior art, the method of ameliorating the uneven temperature distributions at the strip edge portions by adjusting the FM members 20, which is shown in FIGS. 1 to 3, makes it difficult to operate the apparatus with its control limit set below .+-.10.degree. C., even if the distance l (see FIG. 2) of protrusion of each FM member 20 is adjusted to the optimum value. This method is therefore unsatisfactory from the viewpoint of quality control. If the distance l is set to a value more than the optimum value so that each FM member is positioned too close to the corresponsing strip edge portion, the temperature is so distributed that the strip edge portions have an extremely high temperature, as shown in FIG. 4 (B), which could even make them melt.
The other methods disclosed in the specifications of Japanese Patent Laid-Open Nos. 1339/1978, 1614/1978 and 92428/1985 are of no practical use because of their requirement of large-scale equipment, or for other reasons.